Magnesite Cements
The magnesite cements, referred to hereinafter, include magnesium oxychloride cements, having a composition defined by nMgO.MgCl.sub.2.mH.sub.2 O. The art deals with compositions in which n=3 and m=1, or in which m=5 and n=13, or the like. The present invention is not limited to a specific composition and includes any cement the composition of which comprises MgO and MgCl.sub.2, and generally molecular water. The magnesite cements also include oxysulfate cements, the composition of which can be described by the formula m'MgO.MgSO.sub.4.n'H.sub.2 O. Various possible values of n' and m' are known in the art, e.g. n'=5 and m'=3. The expression "magnesite cements" also includes mixtures of oxychloride and oxysulfate cements.
Obviously, the structure and compositions of the cements change during the curing or hardening process, in manners that are well known to skilled persons and are discussed in the literature. Whenever magnesium cements are mentioned in this specification and claims, it is understood that reference is made to cured or uncured or both to cured and uncured cements, as appropriate in each case.
In the specification and claims, the expression "magnesite cement" is intended to include both magnesium oxychloride or Sorel cement, magnesium oxysulfate cement, and mixtures thereof.
It should be noted that magnesite cements are not hydraulic cements, like e.g. Portland cements, as they deteriorate on being soaked in water, while the latter tend to harden under similar conditions. Another difference between the cements mentioned above is the fact that the aqueous solutions that are used to prepare the magnesite cements contains high concentrations of salts (MgCl.sub.2 and/or MgSO.sub.4), while the aqueous solution that are used in the preparation of e.g. Portland cement contain low concentrations of salts. These facts, and the fact that the magnesite cements are basically composed of Mg.sup.++ cations, while the Portland cement should not contain such ions in any considerable amount, have a bearing on the chemical and physical properties of these different cements and therefore on the technologies of their preparation and their uses. Namely, an improvement in one type of cement is not necessarily suitable for the other.
U.S. Pat. No. 2,702,753 reviews critically the state-of-art, at that time, to form porous magnesite cements (incorporation into cementitious materials proper fillers or aggregates which are either of lighter specific gravity or in effect are rendered so by being of porous character; including in the cementitious mixtures chemical ingredients which will react to generate gases; operation under pressure substantially higher or lower than atmospheric). U.S. Pat. No. 2,702,753 discloses a method to produce foamed magnesite cements by merely mixing MgO, MgCl.sub.2 and/or MgSO.sub.4, water and wetting or foaming agents, using simple mixers. This patent discloses that various such wetting agents, known and available for other purposes, may be used to obtain the foamed magnesite cements. Only the general chemical structures of the foaming agents, which reduce surface tension when added to water or watery mixtures, are given.
The alleged potential of this promising technology, which is revealed in U.S. Pat. No. 2,702,753, has not yet materialized on large scale operations for the reason that it is not so effective as claimed. That can be deduced from the ensuing art and from the experimental results shown hereinafter.
The method to produce foamed magnesite cements is described in "Magnesium Oxychloride-Based Foam Thermal Insulation: An Initial Investigation"; W. J. Rossiter, Jr. et al; in "Insulation Materials, Testing, and Applications"; D. L. McElroy et al; ASTM; Philadelphia; 1990; pages 38-51. Though the procedure of producing the foamed magnesite cements makes use of foaming agents (cf page 39), but it does not make use of simple agitation devices, as diclosed in U.S. Pat. No. 2,702,753. The technology that is described in this reference to produce the foamed magnesite cements is much more expensive than simple mixing (with respect to the cost of the devices, operation and maintainance costs and much lower throughputs).
In another later U.S. Pat. No. 4,041,929 the technology of producing foamed magnesium oxychloride cements is diclosed. This procedure makes use of frothing agent--"Norgan Expander" which is actually Mg metal plus lactic acid that react, In Situ, during the production of the magnesite cement to form a dengerous gas (Hydrogen), that froths the mixture. Organic resins, including polyacrylic acids, which are added to the formulation of the cements, do not appear to play a role in causing foaming. On the contrary, the use of the frothing agent in this patent indicates that the organic resins did not foam the cement. The experimental section of the present patent application will reveal that indeed polyacrylic acids do not foam the magnesite cements to any practical extent, when it is prepared using simple industrial mixers.
U.S. Pat. No. 4,699,822 differentiate between magnesite cements and hydraulic cements (cf col.6; lines 32-38). It descibes the various ingredients that are being used to produce foamed cements (cf col. 7; lines 1-4). From this description it is clear that acrylic latex and acrylic powders do not cause any substantial and practical foaming of the magnesite cements, and air entraining agents are used therein.
U.S. Pat. No. 4,992,481 discloses the use of polycarboxylic acids and their salts (cf col. 6; lines 15,16), among many other organic polymeric materials, in the production of magnesite cements, but there is no disclosure of any kind that these materials cause foaming. Indeed, there is no basis for that, as will be demonstrated in the experimental section of the present patent application.
U.S. Pat. No. 4,814,014 discloses the use of graft copolymers of polyether backbones and side chains prepared by polymerization of an ethylenically unsaturated monomers, including acrylic acid and similar compounds. Once again, there is no indication that these materials cause foaming or frothing of the cements. U.S. Pat. No. 4,814.014 teaches that only polymeric moieties are involved in the production of the cements.
Magnesite cements are known to deteriorate in water. Numerous patents and papers describe this phenomenon and suggest of how to solve this problem that limits the use of magnesite cements (foamed and unfoamed). The following discussion will demonstrate some of the solutions that have been suggested in the literature:
U.S. Pat. No. 5,004,505 discloses magnesium oxychloride cement compositions and products obtained by mixing magnesium oxide, a magnesium chloride solution, a strong acid and aggregate particles. The resulting products are said to exhibit increased strength and water resistance. The references cited in the said patent are illustrative of the state of the art.
The addition of phosphoric acid and metal phosphates to improve the properties of the magnesite cements is mentioned, for example, in E. I. Ved et al: "Water-resistant magnesia cement based on caustic dolomite"; Budirelni Mater. Konstr. (1) 35-6 (1969) (also C.A. 103399C Vol. 72 (1970)), H. T. Stamboliev: "Magnesia-Zement--Verbesserung der wasser-bestandigkeit durch phosphatzusatz . . . "; Tonind. - Ztg. 100 (1976) Nr. 1. pp. 34-37, and Israeli Patent No. 81807.
The use of sulphur to improve the magnesite cements is reported in J. J. Beaudoin et al "Impregnation of magnesium oxychloride cement with sulphur": Ceramic Bull. Vol. 56, No. 4 (1977), pp 424-7.
EPA 454660 discloses and claims a Sorel cement, said to have improved water resistance, based on mixtures of MgO, MgCl.sub.2 and inorganic fillers, characterized in that it contains water soluble, polymeric or polycondensed synthetic resins, including water soluble polycarboxylic acids. This patent, also, does not mention any foaming or frothing of the magnesite cements during their production.
DE 3832-287 discloses the use of citric acid and oxalic acids to improve the water resistance of magnesite cements. The possibility that these reagents also lead to foaming of the magnesite cements have not been suggested by this reference. Indeed, as will be demonstrated in the experimental of the present patent application, these agents do not substantially cause foaming of the cement.
As is clear from the above discussion, none of the teachings of the prior art is fully satisfactory, from the viewpoint of water resistance and simplicity of operation and flexibility in formulating objects made from foamed magnesite cements of varied densities and cost effectiveness.
We have found out that articles can be made from improved foamed magnesite cements, which are obtained by mixing MgO, MgCl.sub.2 and/or MgSO.sub.4, water and certain organic carboxylic acids and/or their anhydrides and/or their salts that are able to foam these cements. These cements are characterized by their improved physical and chemical properties and by the low cost of their production. The articles made of this improved foamed cements, in the hardened forms, can be used as construction elements, materials for packaging, adsorbents (e.g. ions, inorganic and organic materials), fillers and flame retardants in plastics and composite materials for special applications and for sound and heat insulators.
It is a purpose of the present invention to provide an inexpensive magnesite cement having improved physical and chemical properties which forms foamed bodies.
It is a further purpose of the invention to provide such cement mixtures, which can be produced by utilizing readily available and inexpensive raw materials and simple mixing devices.
Other purposes and advantages of the invention will appear as the description proceeds.